Power conversion circuitry is employed in many applications, such as in portable devices, which utilize an external power source to operate the device and/or to charge internal batteries of the device. In many instances the current drawn from a source may need to be limited according to specifications published for devices drawing power from the connection. For instance, with a universal serial bus (USB) power supply connection, portable devices may be limited to drawing at most 500 mA from a USB 2.0 connection. Similarly, USB 3.0 connections are typically limited to drawing 900 mA of current.
Power conversion circuitry provides power conversion with current limiting using current sensing. Current sensing circuitry may be important in various power conversion devices such as battery chargers, switched-mode chargers, power converters, voltage regulators, etc. Bi-directional current sensing is needed to monitor the current in such devices during forward current mode and reverse current mode. The sensed current information may then be used for regulating the forward or reverse currents in the device, and also for fuel gauging to compute charge transfer in and out of the battery.
Conventional designs of current sensing devices require two independent uni-directional loops to monitor the current in both the forward and reverse directions. FIG. 1 depicts an example of a conventional current sensing circuit utilized in a switched-mode charger according to the prior art. As shown, current sensing circuit 10 includes a USB_IN input power source configured to supply current to a buck/boost power regulator 120.
Circuit 10 includes two independent current sensing loops 102 and 104 for sensing current flow in both the forward and reverse directions respectively. In particular, current sensing circuit 10 includes a first independent loop 102 comprising a forward current sensing transistor SFET_Chg and a first operational amplifier 106, and a second independent loop 104 comprising a reverse current sensing transistor SFET_RB and a second operational amplifier 108.
During forward current mode, forward current flows into the device from the USB_IN input through a front porch FET transistor FP_FET and into the power regulator 120. The feedback loop 102 comprising the forward current sensing transistor SFET_Chg and operational amplifier 106 senses this current and is configured to equalize the drain-to-source voltage Vds of the power transistor FP_FET and the forward current sensing transistor SFET_Chg such that a sensed current “I1” will be a replica of the forward current.
During reverse current mode, current flows out of the power regulator 120 and back into the front porch power transistor FP_FET in the opposite direction. The feedback loop 104 comprising the reverse current sensing transistor SFET_RB and the operational amplifier 108 senses this reverse current and is configured to equalize the drain-to-source voltage Vds of the front porch transistor FP_FET and the reverse current sensing transistor SFET_RB such that a sensed current “I2” will be a replica of the reverse current.
Such circuits however occupy a significant amount of integrated die area, requiring two independent current sensing operational amplifiers 106 and 108, thereby increasing cost and design complexity.